UB if noreturn syscall actually returns

https://github.com/bytecodealliance/rustix/blob/cb01fbe4660844b67fdd4eee2a5f769518f6a655/src/backend/linux_raw/arch/x86_64.rs#L55-L63

https://github.com/bytecodealliance/rustix/blob/cb01fbe4660844b67fdd4eee2a5f769518f6a655/src/backend/linux_raw/runtime/syscalls.rs#L143-L146

## Every syscall may return

***Every syscall may return** with any value* because of seccomp-bpf.

This includes pure syscalls like `getuid` that may return the current uid as 4294967295_u32 (`-EPERM`) or `exit`/`exit_group` returning. While such seccomp-bpf filters will have huge compatibility problems and are unlikely in real-world, they must be handled in a safe way.

In the case of `exit`/`exit_group` there are two safe possibilities that maintain `-> !` to the caller:

1. Infinity loop
```rust
// Note that this is a pseudo example. If the `sys_exit` function is `-> !`,
// Rust is free to remove the loop because returning from `-> !` is UB.
// The loop must be implemented in assembly with a backward jump
// if the `asm!` is marked `noreturn` as return from the `noreturn`-`asm!`
// is already UB.
loop {
sys_exit(code)
}
```
2. Segfault, i.e. `ud2` (x86-64) after the `syscall`.